Achieving bi-lamellar microstructure with both high tensile strength and large ductility in Ti-6Al-4V alloy by novel thermomechanical processing

In this study, a novel through-beta-transus processing followed by intercritical annealing was designed to obtain the bi-lamellar microstructure in Ti-6Al-4V alloy with refined colony sizes, by which both tensile strength and ductility were significantly improved. The colony size obtained in the through-beta-transus processing was 60 mu m, much smaller than the minimum colony size of 130 mu m that can be achieved in the conventional beta processing. The colony refinement was attributed to the decreased size of the grain boundary alpha phase with increased variety of crystallographic orientations, which acted as nucleation sites for subsequent colony structures. By intercritical annealing of the lamellar microstructures in alpha+beta two-phase region followed by water quenching, bi-lamellar microstructures composed of primary alpha lamellae and transformed beta regions composed of fine secondary alpha plates were obtained, maintaining the same colony size as the lamellar precursors. The total elongation of bi-lamellar microstructure significantly improved from 3.4% to 18.6% with decreasing the colony size, while the high yield and tensile strength was independent of the colony size. SEM-EBSD characterization of the bi-lamellar microstructures at interrupted tensile strains clarified that deformation behaviors of the bi-lamellar microstructures after yielding were mainly controlled by micro-shear bands across transformed beta regions, which eventually evolved into micro-cracks at higher tensile strains. It was considered that the strain compatibility accommodated by the differently aligned micro-shear bands formed within different colonies was the main reason for delaying tensile fracture in the bi-lamellar microstructure with the smaller colony size.